basic operation mcdonnell and miller

7/29/2019 Basic Operation Mcdonnell and Miller

1/1812

BASIC SYSTEM OPERATION

heyve been with us for over two hundred years, and

most of the time, theyre so reliable most folks dont

give them much thought. They sit in buildings all over the

world, transferring heat from fuel to water, allowing us to

warm our buildings or complete our processes.

Steam boilers are simple, efficient and reliable. No

machine does a better job of moving BTUs from one

place to another. Weve used them for space heating

since before the United States Civil War in 1861.

Even before the Civil War, we used steam boilers for

industrial processes. Today we use them to run factories,

press clothes, wash dishes, pasteurize milk, sterilize

medical equipment, and to heat entire cities! Their capa-

bilities seem endless.

But despite its simplicity, any steam boiler can run into

trouble if its control system doesnt act properly. If the

energy you put into the boiler exceeds what the boilercan absorb, the boiler can rupture. So you must always

be on guard.

A simple safety relief valve of the right capacity and

relief-pressure setting protects the boiler from over pres-

sure. But over pressure isnt the only thing that can

threaten a steam boiler. There are also the dangers of

dry firing.

Should the internal water level drop too low, the boiler

can burn out. So here too, you must always be on guard.

You see, a steam boiler needs its water to move the heat

away from its metal surfaces. Without the right internallevel of water, heat quickly accumulates. Too much heat

creates a very dangerous operating condition.

Boiler manufacturers have always set up minimum safe

water level requirements for their equipment. Our con-

trols help enforce those requirements in two ways:

By maintaining a minimum safe water level in

the boiler.

By signaling the burner to stop should the water

level drop below that point.

In this brief Systems Guide we will explain how we do

these two very important jobs.

Whats a Normal Water Level?

The proper steam boiler water level varies from manufac-

turer to manufacturer, but generally, we can say that its

normal to start by manually filling the boiler to the two-

thirds-full point on the gauge glass. As the boiler oper-

ates, the water will quickly turn to steam and head out

toward the system (Fig. B).

Steaming takes place at a constant rate of about one-half

gpm per 240,000 BTU/HR (D.O.E. Heating Capacity

T

Gauge Glass Two-Thirds Full

Steam Boiler

Steam Boilers

Fig. A

Fig. B


2/18 13


Rating). This is a law of physics so it doesnt vary from

manufacturer to manufacturer. If youre working with a

boiler with a rating of, say, 1,000,000 BTU/HR, you can

be assured the water is turning to steam and leaving that

boiler at the rate of about two gpm. And its leaving at

speeds measured in miles per hour (sometimes

exceeding 60 mph!). So its very important for your near-boiler piping to be correct. If its not, the fast moving

steam will pull water out of the boiler and create

problems for you in the system and the boiler.

As the water (in the form of steam) heads out toward the

system, the water level in the boiler will, of course, drop.

How far it drops, depends a lot on the size and condition

of your piping system. You see, ideally, the water should

begin to return to the boiler before the boilers internal

water line drops to a critical point. Thats the point at

which the low water cut-off will cut power to the burner, or

an automatic water feeder will open.

Because the water is in the system piping and radiating

during operation, the normal water level becomes a point

thats somewhere in the lower-third of the gauge glass

(Fig. C).

Remember, youre working with a rangeof operation

here, not a fixed point. If the water were to stay at the top

of the gauge glass all the while the burner was firing, you

probably wouldnt be making steam! So dont get too

caught up with the word normal because the only thing

thats normal is that the water level will rise and fall.

Boiler manufacturers, as we said before, doestablish aminimum safe water level for their boilers, however. That

point is usually just out of sight of the bottom of the

gauge glass. Should the water level drop to this point, the

boiler may be in danger of overheating. We have to find a

way to protect the boiler from itself (Fig. D).

All leading authorities and insurance companies

recognize this need. The ASME Code for Low Pressure

Heating Boilers, for instance, specifies, Each

automatically fired steam or vapor steam boiler shall be

equipped with an automatic low water fuel cut-off.The

device the code refers to is what most people in the field

commonly call a low water cut-off. Its job is to stop theburner and protect the boiler.

What Causes a Low Water

Condition?ecause its an open system, some evaporative water

loss is normal for a steam system. How much

depends on the size and condition of the system. If

youre losing too much water, however, its time to begin

troubleshooting. There are many places to look.

B

Minimum SafeWater Level

Gauge Glass One-Third Full

Gauge Glass Minimum Safe Water Level

Fig. C

Fig. D


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Here are a few good places to start:

The air vents are dirty, not seating properly, and

passing steam to the atmosphere.

Someone left the boiler blowdown valve partially open.

Someone, for whatever reason, has been drawing

hot water from the boiler.

The relief valve has discharged.

The condensate pump isnt working as it should.

The float may have come loose.

The condensate may be too hot to pump.

(Check those steam traps!)

Improper near-boiler piping may be throwing water

up into the system, or causing the waterline to tilt

during operation.

The wet returns may be leaking. (Always suspect

anyburied pipe).

A check valve may be stuck closed or partially closed.

The boiler may be foaming and priming.

Check the pH of the water. It should be between

7 and 9.

Check the condition of the water. Dirty water will

prime and foam.

Check the burners firing rate. Over-firing can

cause priming.

The pipes may not be properly pitched.

The automatic feeder may not be working properly.

Its chamber may be filled with sediment.

Its feed line may be clogged.

All of the condensate may not be returning from the

system (a common problem with process applications).

The boiler metal may be corroded and leaking at

the water line.

Flood the boiler to its header to check for leaks.

Good troubleshooters take the time to look over the

entire system before deciding whats wrong. Take the

time to do it right, and you ll be the person with the

answers.

Watching the Water Levelhe best way to prevent overheating damage to a boiler

is to stop the burner if the water level falls too low. Thisis the low water cut-offs job. There are several types of

low water cut-offs you can use. Lets look at them.

Float Operated Low Water Cut-Offs

Float operated low water cut-offs have been around

(Fig. E) since the 1920s and have earned a reputation

worldwide for reliability. Usually, youll mount this type of

low water cut-off directly in the boilers gauge glass

tappings. We make quick hook-up fittings for these units

to simplify installation.

T

Series 67 Float Type Low Water Cut-Off

Fig. E


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The water level in the low water cut-offs chamber will

mimic the water level in the boiler. As the water level

drops in the boiler during steaming, the level in the

chamber, and the cut-offs float drops with it. Should the

float drop to the boilers critical low water cut-off point, the

float will trip an electrical switch thats wired in series with

the burner. The burner instantly stops firing. It will stay offuntil the water level rises to a safe operating point.

This happens when the condensate returns from the

system or when an automatic water feeder or a boiler

attendant adds water to the boiler. When the level

reaches a safe position, the low water cut-off will make its

electrical connection and the burner will restart.

When a steam system is well balanced, the low water

cut-offs job is to stand by and wait. The situation we just

described suggests that theres something out of balance

in that system. Well look at this again in a few minutes.

Probe and Float Type Built-In Low Water Cut-Offs

There are some jacketed boilers that dont easily accept

quick hook-up fittings. These boilers will often have a

tapping for a built-in low water cut-off. These built-in units

do the same thing as the external units we just looked at,

but instead of being in a chamber, the built-ins are right

inside the boiler where they can sense the water level

directly.

We make two types of built-in low water cut-offs:

Probes The boiler manufacturer will specify the point

where theyd like to have this type of low water cut-offinserted. It will usually sit just below the water line, at a

point above the boilers crown. A probe uses the boilers

water to complete an electrical circuit past an insulator

(the center portion of the probe) back to a ground (the

threaded portion of the probe). As long as water covers

the probe an electronic go signal will travel to the

burner. When water drops off the probe for a continuous

ten seconds, an electronic stop signal goes to the

burner, shutting it down and protecting the boiler from a

low water condition.

At ITT McDonnell & Miller, we manufacture several

different types of probe low water cut-offs to meet any ofyour job applications (Fig. F).

One of those applications might involve the boilers water

level. The water capacity of todays boilers is

considerably less than that of boilers from decades ago.

Along with this, the water level operating range of todays

boilers is smaller. Further, the amplitude of surging water

levels is increasing. As a result, the low water cut-off

must be smart enough to recognize these variations

and react appropriately. We have done this by

Series PS-800 Probe Type Low Water Cut-Off

Fig. F


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incorporating delay features in the probes operating

logic. These include a delay on break feature (DOB)

which keeps the burner lit for 10 seconds after water

leaves the probe. This minimizes the effects of a surging

water line. Another additionthe delay on make feature

(DOM)allows an additional feed time of 15 seconds

once water comes in contact with the probe. Thisminimizes rapid burner and feeder cycling by slightly

elevating the water level so that water lost to steaming

will return (in the form of condensate) before the water

level drops below the probe.

Float Type In operation, these are similar to the

external, float operated low water cut-offs we looked at

before. The difference is that instead of sensing a

duplicated water level outsidethe boiler, these units

sense the level directly inside the boiler.

We make them for you in five mounting-barrel sizes

(Series 69) to accommodate different boiler insulationthicknesses. When you select a built-in, float type control

make sure it fits as far as possible into the boiler, without

the float shield coming contact with the boiler.

When a low water cut-off stops a burner, it also stops the

entire heating system. Nothing will happen until the water

in the boiler returns to a safe operating level.

While this is very good for the boiler, it may not be the

best thing for the system. If the heat in the building is off

for too long a time, water pipes may begin to freeze.

This is where automatic water feeders come in. An

automatic feeder will maintain a safe minimum waterlevel in the boiler and keep it operating, even if the

system is leaking. It keeps the system operating

automatically until you can make the repairs.

Combination Low Water Cut-Offs and

Automatic Water Feeders

Two of our most popular and versatile feeders are the

Uni-Match and the 101A (Fig. G and H). These are ideal

for use in residential or small commercial applications.

They are versatile in that they are compact and they are

easily installed to operate with either a probe type OR afloat type low water cut-off. These feeders are always

ready to add water when given the signal from the low

water cut-off. The benefits they offer are the convenience

of not having to manually add waterand most

importantlythey will protect the boiler from a dry fire

condition by maintaining a safe minimum water level in

the boiler should a system leak occur.

If you use a mechanical automatic water feeder, you can

keep your burner operating even during a power failure.

Uni-Match Water Feeder

Series 101-A Water Feeder

Fig. G

Fig. H


6/18 17


A mechanical feeder can also protect a boiler (Fig. I)

should a fuel-regulating device malfunction, causing the

burner to lock in and stay there. Or suppose someone

jumps-out a control, putting the burner on continuous

operation. A mechanical automatic water feeder will

continue to feed the boiler whenever the level drops to

the feed point.

Under normal circumstances, the electrical low water cut-

off (the second part of the feeder/cut-off combination) is

always standing by, ready to shut off the burner should

something go wrong with the automatic feeder.

An automatic water feeder doesnt feed at the two-thirds

full point on the gauge glass. You set this by hand when

you first start the system. As we said before, the normal

level will range up and down as the system operates. An

automatic feeder will maintain a safe minimumwater line

only. By doing this, it will lessen the possibility of human

error.Consider this. A boiler attendant might put too much

water in a steam boiler. He doesnt have an automatic

feeder and hes tired of checking the water level every

day so he fills the boiler to the two-thirds full point while

its operating. When the condensate returns, the boiler

floods. By adding water the attendant has limited the

boilers steam-making space. Without enough room to

break free of the water, the steam will now carry water up

into the system piping. This leads to higher fuel bills,

uneven heating, water hammer, scale formation in the

boiler and burner short-cycling. Suddenly, problems

plague this system, and no one is sure why.Automatic water feeders help you avoid these problems.

They watch that water level, maintaining a safe minimum.

They allow the boiler water line to rise and fall naturally

through its normal operating range.

How a Feeder/Cut-Off Combination Works

During Normal Operation This is how a McDonnell &

Miller feeder/cut-off combination looks on a steam boiler

(Fig. J). Notice how we have it installed well below the

boilers normal start-up operating range (thats about

two-thirds up the gauge glass). We don t want it to feed

while the water is out in the system as steam.

Remember, the automatic water feeder is there to

maintain a safe minimum water line, not a normal, start-

up water line.

As you now see it in the drawing, the feeder is closed

and the burner is firing. The boiler is working, sending

steam out to the building, and both the automatic water

feeder and low water cut-off are standing by.

The Feeder Opens If the boilers water line drops to the

feeder/cut-off combinations feeder-operating point (which

is very near the bottom of the gauge glass) (Fig. K), the

feed valve will open mechanically and add water to the

BurnerOn

City Water Supply

ReturnMain

Feeder Cut-OffCombination

Steam Boiler

City Water Supply

ReturnMain


Steam BoilerBurnerOn

Normal Operation

Feeder Open & Burner On

Series 47-2 Combination Mechanical

Water Feeder/Low Water Cut-Off

Fig. I

Fig. J

Fig. K


7/1818


boiler. How much water will enter the boiler depends on

several things, but there will always be enough to keep

the boiler operating at a safe minimumwater level. Once

it has added the right amount of water, the feeder closes.

While this is happening, the burner continues to run

because the feeder keeps the boiler from dropping to itslow water cut-off point.

The Low Water Cut-Off Stops the Burner But suppose

something happens and the automatic water feeder cant

keep up with the rate at which the boiler is losing water.

Suppose, for instance, that a pipe breaks or someone

opens a boiler drain, causing the boiler to suddenly lose

water. Should this happen, the water level will drop to a

preset point, and the automatic feeder/cut-off

combination will instantly cut power to the burner,

shutting it down and protecting the boiler from a dry-firing

condition. Though the burner is off, the automatic feeder

will continue to add water to the boiler in an attempt torestore a safe minimum water level (Fig. L).

As you can see, a combination mechanicalwater feeder

and electricallow water cut-off provides you with boiler

protection even if the power fails or something goes

wrong in the burner circuitry.

Combination Water Feeders and Low Water

Cut-Offs for Larger Boilers

As we said earlier, all steam boilers evaporate water at

the rate of one-half gpm per 1,000 square feet EDR

(240,000 BTU/HR). To satisfy a larger boiler s needs, anautomatic water feeder must be able to match the boilers

higher steaming rate. If the feeder cant keep up, the

burner will suffer from nuisance low water shutdowns. To

avoid this problem, we make automatic feeder/cut-off

combinations with wider flow orifices to meet the special

needs of larger boilers. The operation of these larger

units is the same as the ones we just looked at. The key

difference is the increased flow rate (Fig. M).

Once the larger feeder/cut-off combination satisfies the

boilers minimum water line needs, it has to be able to

close against the force of the city water pressure moving

through that extra wide orifice. This calls for considerable

float and lever power, and it explains why our feeder/cut-

off combinations for larger boilers are bigger than those

for smaller boilers. Weve carefully engineered them to

get the maximum closing force in the space we have to

work with. This ensures the unit will close tightly once its

done its job (Fig. N).

Codes call for larger boilers to have their gauge glasses

mounted on water columns, rather than directly into the

boiler. Consequently, we make our larger automatic

BurnerOn

ReturnMain

City Water Supply

Steam Boiler


Large Boilers

Series 51-2 Mechanical Water Feeder

City Water Supply

ReturnMain


Steam BoilerBurner

Off

Feeder Open & Burner Off

Fig. L

Fig. M

Fig. N


8/18


9/1820


condensate receiver. When the water level inside the

receiver reaches a certain point, an electrical float switch

turns the pump on. The pump quickly moves the water

out of the receiver and back into the boiler.

Steam boilers served by condensate pumps also need

low water protection, and our low water cut-offs serve thatpurpose well. You can also use an automatic water feeder

or a combination feeder/cut-off on these systems. But

before you do, make sure the system is well balanced.

What we mean by well balanced is that the condensate

pump should be able to return the water to the boiler

before the boilers water level drops to a point where the

low water cut-off or automatic feeder goes into action.

If the automatic water feeder adds water to the boiler (to

maintain a safe minimum operating level), and thenthe

condensate pump returns its water to the boiler, the

boiler will most likely wind up with too much water. This

excess water limits the boilers steam making space.Without enough room to break free of the water, the

steam can carry water up into the system piping. That

leads to higher fuel bills, uneven heating, water hammer,

scale formation in the boiler and burner short cycling.

So before you use an automatic water feeder on a steam

boiler thats served by a condensate pump, check to see

if the system is well balanced. It should run through its

cycles without going off on low water. In other words, the

condensate pump should balance the flow of water back

into the boiler before the level drops to the critical, low

water point. Keep in mind that a system with a

condensate pump can become unbalanced if the returnsclog with sediment or if any steam traps fail in an open

position.

Good troubleshooters always keep their eyes wide open.

Steam Systems with Boiler-Feed Pumps

If you have a system where some steam is going for

process (meaning, it wont be coming back), or if your

system isnt well balanced, you should consider using a

boiler feed pump instead of a condensate pump.

A boiler feed pump serves the same purpose as a

condensate pump (Fig. Q). It provides the push thewater needs to get back into the boiler. The difference

between a condensate pump and a boiler feed pump,

however, lies in the way we control the two units. Instead

of having an electrical float switch inside the condensate

receiver, a boiler feed pump takes its orders from a

McDonnell & Miller pump controller mounted directly on

the boiler.

The pump controller has two switches. The first switch

(set at the higher of the two levels) operates the boiler

feed pump. When the boiler needs water, the pump

SteamBoiler


Pump Controller

City WaterSupply

Boiler Feed Pump Make-Up WaterFeeder

BurnerOn

BoilerFeedTank

BurnerOn

SteamBoiler

City WaterSupply

Boiler Feed Pump

Pump Control andLow Water Cut-Off

Make-Up WaterFeeder

BoilerFeedTank

Single BoilerFeeder/Cut-Off CombinationUnbalanced System

Single BoilerUnbalanced System

Fig. R

Fig. Q

controller recognizes the need and starts the pump.

When the boiler water returns to the proper level in the

gauge glass, the pump controller stops the pump.

Should the pump not be able to keep up with the boilers

need for water, the pump controller will sense this as

well. The second switch (set at the lower of the two

levels) will cut the electricity to the burner and protect the

boiler from a low water condition (Fig. R).

Feed water enters the system through a make-up water

feeder in the boiler feed pumps receiver. If you wish, you

can add a feeder/cut-off combination to operate at a level

a bit lower than the pump controller. This will give you a

mechanically operated feeder, which will act as a back-

up should something go wrong with the pump controller.

It will also give you a secondary low water cut-off. Its like

having a belt andsuspenders for your boiler!


10/18 21


Meeting the Needs of Systems

with Multiple Steam Boilers(Fig. S, T U)

he boiler on the right may be a stand-by to the boiler

on the left. Every week or so, a boiler attendant

might switch them, making this one the operating boiler

and the other the stand by.

Its a good idea, one weve used for years in larger boiler

rooms. By having more than one boiler, each is able to

supply the entire needs of the system. Your chances of

getting caught without steam are much less.

Some systems have multiple steam boilers. The idea

here is to let several boilers join forces to meet the total

needs of the system. The goal is energy conservation.

You steam all the boilers on start-up, and then shut a fewdown after youve heated the system and satisfied the

piping pick-up load. In other words, you put the system

on simmer after youve heated it completely.

Steam systems with more than one boiler often have

problems if the installer fails to realize that steam is

dynamic and not static. By this, we mean that steam is

always moving veryquickly from the boiler to the system,

and as it moves, it loses pressure. And since one ounce

of pressure represents a water column 13/4 in. (45mm)

high, the slightest difference in pressure between any

two boilers interconnected on their return sides can make

a big difference in the individual water levels.

A slight burr in a pipe or fitting can create a drop in

pressure. You can never tune two burners to produce the

same flame. One boiler will always be closer to the

system take-off than another. These things speak loudly

for proper piping and thoughtful management of the

boiler water line so thats what well look at next.

T

Multiple Boiler Systems with a Boiler Feed

Pump and Motorized Return Valves

Here we have two boilers served by a single boiler feed

pump (Fig. S). One boiler may be a stand by to the other

or they may be sharing the total load. For piping

purposes, wed handle either application the same way.

Notice how the condensate returns are independent.

Each flows from the boiler feed pump receiver to the

boiler through a motorized valve. This is an important

detail. If you were to interconnect the returns, the water

from one boiler would flow into the other.

Steam Moves Remember, steam is dynamic, not static

Water doesnt seek its own level when the steam is

moving out of the boiler. The slightest difference in firing

rate or piping pressure drop between the two boilers will

cause one to flood and the other to shut down due to a

low water condition. This is why those independentreturns are important. Were using motorized valves on

this installation (Fig. S) to isolate one boiler from the

other. When either boiler needs water, the McDonnell &

Miller pump controller on that boiler will drop to a point

where it will close the higher of its two switches. That

switch will power that boilers motorized valve, causing it

to open. When its fully opened, the motorized valve will

trip an end switch and start the boiler feed pump. Water

will flow only to the boiler that needs it. The float in the

pump controller will sense the rising water. When the

water reaches the proper level, the pump controller will

break the electrical connection to the motorized valve.

The valve will begin to close, shutting off the boiler feed

pump as it does.

As you can see, when we pipe multiple boilers this way it

doesnt matter how big or small each is. The boiler feed

pump, although sized for the totalneeds of all the boilers

will satisfy the needs of each in turn, no matter what size

Keeping the Water Flowing Weve installed a make-up

water feeder in the boiler feed pumps receiver tank. Its

job is to maintain a minimum water line in the tank so the

pump will always have a reservoir from which it can draw

feed water. In this system, all the water will enter the

boilers through the boiler feed pump. If, for any reason,the boiler feed pump cant keep up with the boilers rate

of evaporation, the water line in the boiler will drop. The

lower switch in the McDonnell & Miller pump controller

will stop the burner.


11/1822


BurnerOn

BurnerOn



MotorizedValve MotorizedValve

SteamBoilerNo. 1

SteamBoilerNo. 2

City Water Supply

Make-Up WaterFeeder

Boiler Feed Tank

BoilerFeed Pump

Multiple BoilersBoiler Feed Pump and Motorized Valves



MotorizedValve

MotorizedValve

BurnerOn

BurnerOn

SteamBoilerNo.2

SteamBoilerNo.1

City Water Supply

Boiler Feed Tank

BoilerFeeder Pump

Make-Up WaterFeeder

Pump Controller Pump Controller

Multiple BoilersBoiler Feed Pump, Motorized Valve & Water Feeders

ElectricProportioning

Regulator

ElectricProportioning

RegulatorHigh Water

Cut-Off and AlarmHigh Water

Cut-Off and Alarm

SteamBoilerNo. 1

SteamBoilerNo. 2

Motor OperatedProportioning Valve

Motor OperatedProportioning Valve

BalancingValve

BalancingValve

Boiler Feed Tank

City Water Supply

Make-Up WaterFeederBoiler Feed Pump

Pump Return By-passWith Balancing ValveOrifice Or Relief Valve

Multiple BoilersBoiler Feed Pump, Electric Proportioning Regulators and Motorized Valves

Fig. S

Fig. T

Fig. U


12/18 23


If you find the pump suddenly cant keep up with the

boilers needs, check the temperature of the returning

condensate. As thermostatic radiator steam traps and

end of main F&T traps age and fail, they pass steam into

the returns. That can make the condensate hot enough

to flash when it hits the pumps impeller. Boiler feed

pumps cant move water once it has flashed to steam.

The pump will turn and cavitate, but it wont satisfy the

boiler.

Ideally, in a low pressure steam heating system, the

condensate in the pumps receiver shouldnt be hotter

than 180F (82C).


Pump, Motorized Return Valves and Boiler

Water Feeders (Fig. T)

This is the same system we just looked at, except weve

added a combination automatic water feeder and lowwater cut-off to a point just below the pump controller.

The feeder/cut-offs job will be to add water mechanically

to the boiler should something happen to the boiler feed

pump (for instance, if its cavitating because the return

condensate is too hot).

Think of the feeder/cut-off as a back-up device to keep

the boiler in operation should something go wrong

elsewhere. The low water cut-off will back up the pump

controllers primary low water cut-off should something

go wrong there, or if the feeder cant keep up with the

boilers rate of evaporation for some reason.


Pump, Motorized Return Valves and Electric

Proportioning Regulator (Fig. U)

Here were controlling the water lines with electric

proportioning regulators. Were matching the incoming

feed water to the exact amount of water thats leaving assteam. By doing this, were able to maintain a precise

water line in both boilers and take advantage of each

boilers full steaming space.

There are times when steaming loads will vary

tremendously. This is especially true of steam heating

systems in larger buildings. We often set up these

buildings to operate on outdoor air temperature sensors

and night set-back devices. When the system first starts

in the morning the boilers will steam longer than they will

during the day when the pipes and radiators are hot. This

is also true of seasonal operation when you run the

heating system less often.

This is when proportioning regulators can make a big

difference. By closely monitoring the water line,

regardless of varying system conditions, you improve the

quality of steam leaving the boiler and allow the system

to operate more efficiently.


13/18

Receiver Tank Controlf you size a boiler feed pumps receiver properly it will

be able to hold the right amount of water to keep the

boiler operating during the start-up cycles. It will also be

able to receive the returning condensate without

overflowing.

Receiver sizing is more an art than a science. You have

to look closely at the entire system to figure out how long

it will take the condensate to return from the building.

There are many variables to consider: The type and

condition of steam traps, the pitch and cleanliness of

steam mains and returns, the pipe insulation or lack of it,

the shape of the building and how people use it.

There are also the times when youll have to deal with

condensate transfer pumps, or maybe a vacuum/con-

densate pump. These pumps collect and relay return

water back to the boiler feed pump. There are manythings that can affect how quickly these secondary

pumps move condensate back to the primary boiler feed

pump. You have to consider them all when youre sizing

a feed pump receiver.

One thing will be a constant, however. There must

always be enough water in the receiver for the boiler to

draw from during the start-up cycle (the time between

initial steaming and the return of condensate from the

building). A McDonnell & Miller make-up feeder, set at

the one-third full point on the receiver tank, will meet the

boilers needs during this crucial start-up time. Lets take

a closer look at these.

Receiver Tank Make-Up Water Feeders

Here, weve mounted a McDonnell & Miller make-upwater feeder on a one-inch NPT equalizing line thatextends from the top of the tank to the bottom. The level

in the feeders chamber will be the same as the level inthe tank. As the pump moves water out of the tank and

into the boiler, the float inside the feeders chamber willopen and constantly replenish the tanks reservoir.

Weve designed our feeders with the right amount of float

and lever power to close tightly against city water

pressure. This ensures that there will always be enoughtank space to receive the returning condensate without

having it overflow.

If the tank youre using doesnt have tappings for an

equalizing line, you can use our internal feeder (Fig. V).

As you can see, it mounts directly inside the tank and

feeds water through its integral strainer. We make this

unit with two flange sizes for both new and retrofit

installations.

24


I

Condensate Receiver Tank

City Water Supply

Model 21

Make-Up Water

Feeder

Make-Up Water Feeder

Fig. V


14/18 25


City Water Supply

Make-Up WaterFeeder

BoilerFeed Pump

Low Water

Cut-OffCondensate Receiver Tank

Low Water Cut-Offon Receiver Tank

City WaterSupply

Cond. Return

Needle Valve

Diaphragm Valve

Check Valve

Model 25AMake-Up Water

Feeder

Large Condensate Receiver Tank

Make-Up Water FeederUsed as Pilot Valve

Large Condensate Receiver Tank

Motorized ValveCity Water

Supply

Model 93Controller

Make-Up Water Feederand Motorized Valve

A Make-Up Water Feeder Used as a Pilot Valve(Fig. W)

When you have multiple boilers, the boiler feed pump has

to be able to meet the needs of allthe boilers should they

need water simultaneously. During the start-up cycle, the

draw from the feed pumps receiver can be very heavyand the make-up feeder has to be able to match that flow.

When we run into this situation, we often use a make-up

water feeder as a pilot valve to operate a high capacity

diaphragm valve with dead-end service. When the

feeder opens it signals the diaphragm valve to snap into

action. The larger valve quickly maintains the receiver at

the one-third full point. Once the feed pump shuts off the

dead-end service valve closes tightly to prevent over

filling. If returned condensate fills the receiver, the feed

valve, of course, stays closed. This piping arrangement

also gives you a lot of freedom because you can put the

diaphragm valve in a remote location, if youd like, foreasier service.

A Make-Up Water Feeder with a Motorized Valve(Fig. X)

Heres another way you can quickly fill the receiver. Use

a McDonnell & Miller controller to sense the tanks water

line. As the level rises and falls, the controller will

electrically operate a high capacity motorized valve. This

is another piping arrangement that gives you a lot of

freedom. You can place that motorized valve anywhere

youd like.

Low Water Cut-Offs for Receiver Tanks(Fig. Y)

Theres always the possibility for human error on any job.

For instance, suppose someone decides to turn off the

water supply to your receiver tank. The pump controller

on the boiler will still start the pump, but once the receiver

goes dry there wont be any water to pump because of

the closed valve. Or suppose the building loses water

pressure and the feed pump suddenly finds itself moving

more water than the water feeder can replace. If the

pump runs dry, it will cavitate and its mechanical seal will

quickly heat and break. That leaves you with a costlyrepair and system down time.

If you install a low water cut-off in an equalizing line

around the tank, the cut-off will protect the pump no

matter what happens.

Fig. W

Fig. X

Fig. Y


15/1826


BurnerOn

Compression Tank

ColdWater

SupplyLow Water

Fuel Cut-OffSeries 63

Test-N-Check

Valves

Blow DownValve

Hot Water Boiler

ASME ReliefValve

Series 850 or 900Low WaterCut-Offs

ReturnMain

Supply Main

Hot Water Boiler

Hot Water Boilersow water protection isnt just for steam boilers. Hot

water boilers face the same perils of overheating

damage if the water line drops too low. Many people

dont think of this as often as they should because hot

water boilers serve closed systems. They havepressure reducing valves that are supposed to feed

water automatically should a leak develop.

The truth, however, is that a pressure reducing valve is no

substitute for a low water cut-off. Pressure reducing, or

feed valves, often clog with sediment and wind up not

feeding at all. A buried pipe can corrode and spring a leak

that flows faster than a feed valve can satisfy. Relief

valves can pop and, while dumping water at a great rate,

actually prevent the feed valve from operating.

Lets take a closer look at how we can protect these boilers.

Hot Water Systems (Fig. Z)

As we said, the things that affect steam boilers also affect

hot water boilers. If you run them with too much water the

relief valve will open. If you run them with too little water

theyll overheat and suffer damage.

A low water cut-off is the only sure way of protecting a hot

water boiler from sudden loss of water. The ASME boiler

code recognizes this by requiring all hot water boilers of

400,000 BTU/HR or more input to have low water fuel cut-

off devices.

ASME doesnt call for low water cut-offs on smaller,residential boilers, but we think allhot water boilers,

regardless of their size, must have protection. However,

the International Mechanical Code requires low water cut-

offs on ALL hot water and steam boilers. ITT McDonnell

& Miller make several devices, both float and probe type,

that protect and meet the needs of any boiler whether its

cast iron, steel, or copper construction (Fig. AA, BB, CC).

Hot water systems regularly lose water through faulty air

vents, loose valve stem packing, cracked boiler sections,

loose nipples, corroded pipes, broken or loose pump seals,

leaking gaskets, dripping relief valves, to name just a few

places. Most installers depend on their pressure reducingor feed valve, to replace the lost water automatically. But

feed valves often clog with sediment, especially in hard

water areas. And its very easy to close the supply valve to

a feed valve and forget to open it again.

Fig. Z

On systems with buried pipes (say, a radiant heating

system) a feed valve will open if a pipe breaks. It will feed

fresh water continuously until it either clogs (and stops

feeding) or destroys the ferrous components of the

system with oxygen corrosion. A simple feed valve can

wind up costing a lot more than its purchase price. This is

why major suppliers of feed valves, such as ITT Bell &

Gossett, recommend you close the feed valve once

youve established your initial fill pressure.

This is also why we strongly recommend you use a low

water cut-off on every hot water boiler. Feed valves are

not a substitute for low water cut-offs. They can't protect

your boilers from a low water condition. Feed valves are

fine for filling the system initially, and for helping you vent

air from the radiators. But once the system is up and

running, you shouldnt look to them for protection.

L


16/18 27


Series PS-851 Probe Type Low Water Cut-Off

Series 67 Float Type Low Water Cut-Off

Series RB-24 Probe Type Low Water Cut-Off

Over firing

There are times when hot water boilers dont lock-out on

safety. Whether by control failure or human error, things

go wrong. And when they go wrong in a hot water

heating system, the water temperature can rise quickly to

a point where the compression tank cant take up theexpansion of the water. This causes the relief valve to

discharge.

When the relief valve opens, theres a sudden drop in

system pressure. The water, which at this point is

probably much hotter than 212F (100C), will flash into

steam. This is why ASME insists that relief valves for hot

water boilers carry steam-discharge ratings.

If a feed valve doesnt open to replace this rapidly exiting

water, a low water condition will quickly result. The only

thing that can protect the boiler at this point is a low water

cut-off. The feed valve cant protect the boiler because itstypical setting is 12 psig (.83 bar). In other words, the

system pressure must drop below 12 psig (.83 bar)

before the feed valve will open.

The trouble is that while the relief valve is open and

flashing steam to atmosphere, the internal system

pressure never drops anywhere near 12 psig (.83 bar). A

relief valve with a 30 psig (2.1 bar) setting, for instance,

will open at 30 psig (2.1 bar), and close again when the

pressure drops to about 26 psig (1.79 bar). The result is

a loss of water with no make-up. Repeat this cycle

enough times and the boiler will be in a dangerous, low

water condition. Keep in mind, steam exerts pressure. Itcan easily fool a feed valve, and thats why feed valves

offer very little protection at all against low water.

Fig. AA

Fig. BB

Fig. CC


17/1828


CirculatingPump

Cold Water Supply

Pressure ReducingValve with Built-inCheck Valve

Return fromSystem

Relief Valve

Compression Tank

Flow

Flow Switch Zone ControlValves

Flow Bypass Line

Compression Tank

City WaterSupply


Model 247-2, 51-2,51-S-2, 53-2

WaterLevel

Return

Test-n-Check

Valves

Blow DownValve

BurnerOn

ASME ReliefValve

Supply Main

Series 850 or 900Low WaterCut-Offs

Return

Series FS4-3 Flow Switch

(shown without paddle)

Hot Water Boiler

Copper Fin Tube Boiler

Feeder/Cut-Off Combinations for Cast Iron

and Steel Hot Water Boilers (Fig. DD)

To protect a boiler from dry firing, the low water cut-off

must located above the boilers crown. After the low

water cut-off shuts off the burner, you should have a way

to add water to the system to ensure the crown staysunder water.

A combination water feeder and low water cut-off can do

this for you. If you position the feeder above the boilers

crown, it will mechanically feed water if the level should

drop to that point. This is an important consideration

because, even if the electricity is cut off, its possible for

the firing cycle to continue if the fuel feed valve is

mechanically locked open. The combination units cut-off

switch will act as a back-up to the primary low water cut-

off, providing the boiler with additional protection.

Protecting Copper Fin Tube Boilers (Fig. EE)Copper fin tube boilers move heat from the flame to the

water almost instantly. This type of boiler depends on the

proper flow of water across its heat exchanger to move

the heat quickly out of the boiler and into the system.

Should flow stop while the burner is operating, heat will

quickly build and cause the water in the heat exchanger

to flash into steam. This condition is similar to a dry firing

in a cast iron or steel boiler.

A McDonnell & Miller flow switch, installed on the copper

fin tube boilers hot water outlet, protects it from this

danger (Fig. FF). The burner cannot fire unless water ismoving across the flow switch. When the flow stops, for

whatever reason, the McDonnell & Miller flow switch

immediately cuts electrical power to the burner and

protects the boiler from overheating.

Fig. FF

Fig. DD

Fig. EE


18/18


Series 250 Pressure Relief Valve

Series 260 Pressure Relief Valve

Pressure Relief Valves(Fig. GG, HH)

ood engineering practice calls for every hot water

boiler to have a pressure relief valve. This spring-

loaded valve must be able to release the boilers entire

load at the boilers maximum operating pressure.

Here are some things that can cause a relief valve to

open in a hot water heating system:

The automatic feed valve fails, allowing higher than

normal pressure to enter the system.

Someone leaves a hand bypass line open after filling

the system.

Someone hydrostatically tests the system at a

pressure greater than the relief valves setting.

The air cushion in the diaphragm type compression

tank doesnt match the systems static fill pressure.

Keep in mind, most tanks come from the factoryprecharged at 12 psig (.83 bar). If the system needs

more than 12 psig (.83 bar) pressure, you have to add

more air to the tank, and you have to do this while you

have the tank disconnected from the system.

The compression tank may be too small for the system.

The boilers aquastat is in a well without heat

transfer grease. When this happens, the boilers

temperature will quickly exceed the aquastats

setting, causing rapid rise in system pressure.

The circulator may be on the return side of the

system with the compression tank at its suction. If itis, the circulators head pressure will appear inside

the boiler as a net increase. It may be enough to

open the relief valve.

The burner limit may be jumped-out or stuck in a

manual position.

The main thing to keep in mind when youre

troubleshooting this one is that relief valves pop when

any of these three things happen:

The compression tank loses its air cushion

The system takes on more water.

The system temperature increases.

Think methodically, and keep your eyes wide open!

e hope this Basic System Operation Guide has

given you insight into the systems on which youre

now working or will face in the future. We welcome any

questions or comments you may have about the Guide,

or about our products.

Thanks for your support, and for your continuing business.

Fig. GG

Fig. HH

G

W

basic operation mcdonnell and miller

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